I am looking for some sort of push-pull component, where I can for example have three wires, 5v, 0v, and then a wire that if I send 5v down, it will push a cylinder fully, 2.5v - it will push a cylinder half way, and 0v it will have it retracted.

If you can imagine a car engine, this would be like the piston. I want to use it for making a kind of digger arm. I can think of a design using a servo but that wouldn't really be the traditional "hydraulic" way that an arm moves.

Do any of you know of a component like this? I have searched for solenoid components on ebay, maplin, all over the show, but to no luck.

Well, the motor that Jaguarjoe linked to has a 50-slot photo tachometer on it; maybe you can just count the pulses from the tach to determine the extent once you'd retracted it to establish a zero point by hitting a switch or similar.